Q: How did the music teacher get locked in the classroom?
A: His keys were inside the piano!

Explanation:  Well, the word “keys” in this joke has two meanings: 1) Keys are the white and black parts of the piano that your fingers touch when you play the piano; 2) Keys are the pieces of metal that you use to unlock doors.  This joke is funny because piano keys are in the piano, they are part of the piano but if you leave the keys to your room in the piano then you cannot get into you room.

Piano teachers need to keep their keys in their pockets and not in their pianos!